The individual forms of cytochrome P-450 display unique substrate specificity and reactivity profiles toward a variety of drugs and carcinogens. Differences in cytochrome P-450 phenotype may relate to individual differences in sensitivity to certain drugs and susceptibility to carcinogenesis. Monoclonal antibodies (MAbs) to cytochromes P-450 have been used as specific probes for epitope-specific cytochromes P-450 in human liver samples obtained from different individuals. Western blot analysis with antisera or a MAb to rat ethanol-induced P-450 detected a P-450 in human liver homogenates and microsomes. The level of the immunodetectable P-450 varied among livers from different individuals. A human liver P-450 was immunopurified using the MAb and was structurally characterized. In addition, we are developing a radioimmunoassay for human cytochrome P-450 as a rapid, efficient method for screening large numbers of samples from human tissues.